The present invention relates to a disk type brushless coreless DC motor, and more particularly to a disk type brushless coreless DC motor comprising a rotor made up with alternately configured N-S magnetic pole, a stator having a position-detecting sensor and a board on which more than one air cored armature coil is disposed facing the field magnet of the rotor.
And more particularly, the present invention relates to a disk type brushless coreless DC motor in which cogging force is generated by the screw combining the circuit board on which armature coils are disposed with a casing member of the stator and thereby the a dead point could be eluminated.
According to the growing tendency to lighter, thinner, smaller DC motors, efforts have been concentrated to reduce unnecessary parts and members of rotors and stators in DC motors.
As a result of those efforts, a brushless coreless DC motor has been proposed that has dead points in which the rotational torque of the rotor becomes zero because the coil torque characteristic of the armature coil in the rotational state, so such DC motors as are shown in FIG. 1 and FIG. 2 have been provided for dead point elimination.
For an example, in FIG. 1 the motor comprises a rotor made up with a rotor yoke 41 on which field magnet 42 is disposed, and a stator is made up with stator yoke 45, on which armature coil 43 is installed, and a position-detecting sensor, the stator yoke 45 being a specially structured saw tooth shape in cross section.
And in the configuration of FIG. 2, an iron bar 46 is installed for cogging torque generation in armature coil 43, instead of the saw tooth type stator yoke of FIG. 1. The flux distribution produced in the illustrated relationship of rotor field magnet 42 and iron bar 46 in a stationary state is shown in FIG. 3, and the flux distribution around the dead point is shown in FIG. 4.
On the other hand, another method has been suggested in Japanese Laid-open Utility Model Gazette Nos. Showa 61-192674, 61-192676, 62-2367, that eliminates dead points in DC motor by putting a magnetic substance for cogging torque generation at the other side of the armature coil board in a various forms in brushless coreless DC motor. With this method, the cogging torque is to be generated at the position of 22.5.degree. with a rotor with a 4 pole field magnet, 15.degree. with a rotor with a 6 pole field magnet, and 11.5.degree. with a rotor with an 8 pole field magnet; i.e., cogging torque is to be generated at 1/4 position of magnetic pole width. Accordingly the combined torque curve of a rotor in which a 4 pole magnet is attached becomes as in curve (a) in FIG. 5.
This combined characteristic curve represents the ideal state, where (b) represents the torque curve by armature coil, and (c) represents the cogging torque curve.
In the above methods, however, the technique of FIG. 1 involves difficulties in production because it requires a special saw tooth shaped yoke facing the field magnet of the rotor for dead point elimination, so it turned out not to be a desirable method, and also because of a peeling off problem of the coil, occurring in the assembling the process of armature coil relative to the upper face of the rotor yoke, which resulted in an increased error rate.
In the case of the technique of FIG. 2, a specially structured iron bar is to be put and held inside of the air cored armature coil, which involves difficulties in production, so this method also turned out not to be a desirable one. In addition, in the case of the preferred method disclosed in Japanese Laid-open Utility Model Gazette No. Showa 61-192674, 61-192676, and 62-2367, the separate stator yoke is to be specially cut and installed on the back side of a circuit board, so this method also turned out not to be a desirable one because of the complicated structure requiring a separate stator, yoke and accompanying difficulties in production. Especially in this method, insulation between circuit board and stator yoke is essentially required, thus the process becomes complicated with increasing cost.
Thus the prior art described above required special structures for dead point elimination in a brushless coreless DC motor, resulting in an increase in the number of components or complexity, which involved an increase of size and price, so it turned out not to be desirable.